This invention relates to a rechargeable device for storing and, when desired, releasing hydrogen. Among other applications, the device can be used in the energy generation or transportation industries. The U.S. government has rights in this invention pursuant to contract no. DE-AC09-89-SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
Hydrogen burns cleanly in air, producing water as a xe2x80x9cwastexe2x80x9d product. Powering vehicles, machinery or appliances with hydrogen powered engines or fuel cells eliminates the air pollution associated with fossil fuel powered engines.
But the heat value per volume of hydrogen is very low compared to fossil fuels like gasoline. The heat value per volume can be increased by placing the hydrogen under thousands of pounds per square inch of pressure, cooling it to a liquid, or absorbing it into a solid such as a metal hydride. Pressurizing or liquefying hydrogen requires bulky, expensive processing and storage equipment. It can also be dangerous. For instance, if liquid hydrogen is heated, it converts to a gas. This may significantly raise the pressure within its storage device, with possible drastic consequences.
Placing hydrogen in a solid form avoids these problems. Storing hydrogen as a solid has many advantages. For instance, volumetric hydrogen density in a solid such as a metal hydride is fairly high, about the same as liquid hydrogen, making metal hydride a compact storage medium. And binding the hydrogen as a solid means it will not desorb unless heat is applied, thereby improving safety.
Metal hydrides are heavy, however. The gravitational density of hydrogen is very low. This means the amount of energy per weight of metal hydride may be low compared to fossil fuels.
Metal hydride also creates several engineering problems. For instance, metal hydride tends to break down into fine particles that can plug a gas filter. The hydride particles expand and contract upon hydrogen absorption and desorption, respectively. This may cause densely packed metal hydride beds to form that when they expand may damage the container holding the metal hydride. Adding or removing heat is necessary to the hydrogen absorption and desorption processes; hydride powders transfer heat poorly, however.
Attempts to overcome these problems have been made. Some metal powders, like copper or aluminum powders, have been used as a binder to help hold pressed metal hydride powder in compacts. But the compacts expand and degrade over time. Sandwiching metal hydride particles between metal plates to keep the particles in place and improve heat transfer has also been tried. Yet the plates further increase the overall weight of the storage device. They also restrict the volume expansion of the particles. Operational problems eventually develop as the swelling and contraction of the particles begins to affect hydrogen fluid flow and heat transfer through the plates. Performance ultimately degrades.
The present invention overcomes these problems. It provides a rechargeable device to store and, when desired, release hydrogen. A solid storage medium, like metal hydride in a ground particle form, may be used to hold hydrogen. The storage medium is placed within a container. Dividers partition the container into chambers. A matrix, formed from a thermal foam or other appropriate materials and placed within the container, improves heat transfer and holds the solid hydrogen storage medium in separate cells. Although the storage medium may migrate somewhat among cells, the dividers prevent the storage medium from migrating into a different chamber. This helps evenly distribute the storage medium. When metal hydride particles are selected as the storage medium, the cell format avoids excessive particle settling but allows for particle expansion and contraction. The device also provide a modular design so that the total hydrogen capacity is flexible.
The matrix thermally couples to a heat transferring surface. For example, a channel or conduit can be provided for conveying a fluid by which heat is removed or added to the container to cause hydrogen absorption or desorption from the storage medium. Because the matrix is thermally conductive, it fully distributes heat throughout the container or transfers it to the heat transferring surface for removal by a coolant fluid. A heat transferring surface also could be formed by a heated, electrically or otherwise, platen placed in contact with the outer container walls, to the inside of which the matrix may be tightly fitted for good thermal conductivity. When heat removal is necessary, the platen could be cooled by surface radiation, assisted by a fan and/or thermal fins located on the platen side not coupled to the outer container walls.
A port located in the container allows hydrogen to enter in order to recharge the storage medium or exit when hydrogen desorption occurs. A porous filter, positioned within the port, keeps the storage medium in the container, while allowing hydrogen to pass. If the filter extends throughout most of the container it also helps hydrogen fluid circulate to and from the chambers.
In one embodiment, the hydrogen storage device comprises one or more modules. A module may be composed of multiple, interconnected containers, each formed from two roughly concentric pipes that lay horizontally. Attached or fitted tightly to the outside of the inner pipe is the matrix formed from a thermal foam, like an aluminum foam. It substantially fills the space between the two pipes. The thermal foam matrix forms a network of open cells. Metal hydride particles (or another hydrogen storing solid) occupy and migrate among these cells. But the particles cannot pass between dividers that partition the container into chambers providing even distribution while ensuring sufficient space for particle expansion following hydrogen absorption. This may be done by ensuring that the ratio of chamber length to container (inner) diameter ranges between about 0.5 and 2, and preferably is about 1. The space between the two pipes, roughly annular in shape, is sealed on both ends but for a single port through which hydrogen fluid may flow. A porous metal filter may be inserted in the port to prevent the escape of metal hydride particles but allow hydrogen fluid to flow. The filter can extend through substantially the length of the container to allow for better hydrogen fluid flow. Each module has several such containers, whose inner pipes and ports are interconnected.
When hydrogen release is desired, a hot fluid may be circulated through the inner pipes to release the hydrogen. Thermal foam matrixes are tightly coupled to the outside of the inner pipes; each therefore distributes heat through its cells and ultimately to the storage medium. To recharge the storage medium, hydrogen is pumped into the interconnected ports, flows through the open cells and circulates about the storage medium. Hydrogen absorption results; it generates heat, which the thermal foam matrixes conduct to the inner pipes. A coolant fluid conveyed through the interconnected inner pipes removes the heat, thus increasing the efficiency of the charging process.
Storage containers can be formed in virtually any shape. For example, rectangular ducts partitioned into chambers having multiple cells could be used to hold the metal hydride particles. A channel for circulating coolant or hot fluids could surround the duct, pass through it or lie adjacent one or more of its walls to create a surface for adding or removing heat. A thermal foam matrix is thermally coupled to the surface for distributing the added heat throughout the container or delivering heat generated in the container to the surface, for removal.
Another embodiment uses a simple cylinder for the container. A generally xe2x80x9cU-shapedxe2x80x9d conduit inserts into the container for circulating heat transferring fluid that enters through one opening in the conduit and exits from another. The U-shaped conduit is particularly appropriate for a storage device whose length is more than about five times its (inner) diameter. For such storage devices a straight length of conduit or inner pipe may expand or contract with temperature changes; these movements may stress the container or the seals at the intersection of conduit and container. The U-shaped conduit, particularly if its base is separated a slight distance from the container end wall, allows for such expansion and contraction. Preferably, fluid enters the lower leg of the conduit that is positioned near the container bottom. This cools the lower part of the container first during hydrogen absorption. Thus, as the solid storage medium absorbs hydrogen, it will be able to push the top layers up as it expands. Failure to first cool the lower end may slow absorption and stress the container.
Multiple containers, in these or other configurations, can be interconnected into modules; multiple modules can be interconnected to even further increase the total hydrogen storage capacity of the device. For instance, two fluid-circulating conduits for different containers could be coupled so that the same coolant or hot fluid (e.g., water or air) circulates through both. Similarly, their ports can be manifolded together to allow for charging or discharging. Then, of course, several such modules could be interconnected to even further increase storage capacity or facilitate hydrogen release or charging of the selected solid hydrogen storage medium.
Ground metal hydride particles may be selected as the solid hydrogen storage medium. A reasonably priced metal hydride alloy can hold about 1.3% hydrogen by weight. But other materials could also be used. In fact, the percentage weight of hydrogen that can be stored in solid form is expected to increase as present materials are refined or new ones developed. Or the cost of better, presently available but expensive hydrogen storing solids may decline. Those new, better or cheaper storage mediums also can be used with the present invention.
Moreover, numerous material types could be selected in forming the container or its interior cell walls that hold the solid hydrogen storage medium. But material having high thermal conductivity is preferred. Also, surrounding modules or containers with insulation may provide greater control over the hydrogen absorption and release processes since the insulation minimizes the impact of the ambient environment on heat transfer to and from circulating fluids.
Multiple advantages flow from this hydrogen storage device. Some of those include:
A matrix formed from thermally conductive material like a thermal foam that improves heat transfer within the container;
Cells, formed by the matrix, which distribute heat to and from the solid hydrogen storage medium;
Dividers that separate the container into chambers that keep the storage medium evenly distributed among the chambers to avoid the particle expansion problem;
A modular design that permits simple adjustment of the total hydrogen capacity; and
The use of a metal hydride in a ground particle form for the storage medium, which avoids the need for compaction or other treatment, thus lowering the cost.
This hydrogen storage device can be used in the energy and transportation industries. For instance, it can be used as a xe2x80x9chydrogen tankxe2x80x9d for buses, cars, trucks, locomotives, boats or other vehicles presently using internal combustion engines or fuel cells. Additionally, such xe2x80x9chydrogen tanksxe2x80x9d can be used for mobile power sources presently provided by gasoline or diesel fuel generators.
Accordingly, one object of the present invention is to provide a hydrogen storage device using a container partitioned into chambers.
It is another object of the present invention to provide a matrix formed from a thermally conductive material to improve heat transfer within the container. It is yet another object of the present invention to fill the chambers with the matrix to create multiple cells. It is also an object of the present invention to form the matrix from a thermal foam, like an aluminum foam, which will deliver heat to and conduct heat from the container.
It is the further object of the present invention to dispense within the cells a solid storage medium for holding hydrogen.
It is an additional object of the present invention to provide a heat transferring surface, like a channel or conduit for conveying fluid, which delivers heat to or removes heat from the solid storage medium.
It is yet an additional object of the present invention to provide a hydrogen transfer port through which hydrogen fluid may pass during discharging or charging of the hydrogen storage medium.
It is yet a further object of the present invention to provide a filter within the transfer port that passes hydrogen fluid but prevents escape of the storage medium from the container.
Other objects, features and advantages of the present invention will become apparent upon reviewing the remainder of this document, including the description, drawings and claims.